.TH std::map::map 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::map::map \- std::map::map

.SH Synopsis
   map();                                                                 \fI(until C++11)\fP
   map()                                                                  \fI(since C++11)\fP
       : map(Compare()) {}
   explicit map( const Compare& comp,                                \fB(2)\fP
                 const Allocator& alloc = Allocator() );
   explicit map( const Allocator& alloc );                           \fB(3)\fP  \fI(since C++11)\fP
   template< class InputIt >

   map( InputIt first, InputIt last,                                 \fB(4)\fP
        const Compare& comp = Compare(),

        const Allocator& alloc = Allocator() );
   template< class InputIt >

   map( InputIt first, InputIt last,                                 \fB(5)\fP  \fI(since C++14)\fP
        const Allocator& alloc )

       : map(first, last, Compare(), alloc) {}
   map( const map& other );                                          \fB(6)\fP
   map( const map& other, const Allocator& alloc );                  \fB(7)\fP  \fI(since C++11)\fP
   map( map&& other );                                               \fB(8)\fP  \fI(since C++11)\fP
   map( map&& other, const Allocator& alloc );                       \fB(9)\fP  \fI(since C++11)\fP
   map( std::initializer_list<value_type> init,                  \fB(1)\fP

        const Compare& comp = Compare(),                             \fB(10)\fP \fI(since C++11)\fP

        const Allocator& alloc = Allocator() );
   map( std::initializer_list<value_type> init,

        const Allocator& alloc )                                     \fB(11)\fP \fI(since C++14)\fP

       : map(init, Compare(), alloc) {}
   template< container-compatible-range<value_type> R >

   map( std::from_range_t, R&& rg,                                   \fB(12)\fP (since C++23)
        const Compare& comp = Compare(),

        const Allocator& alloc = Allocator() );
   template< container-compatible-range<value_type> R >

   map( std::from_range_t, R&& rg,
        const Allocator& alloc )                                     \fB(13)\fP (since C++23)

       : map(std::from_range, std::forward<R>(rg), Compare(),
   alloc) {}

   Constructs new container from a variety of data sources and optionally using user
   supplied allocator alloc or comparison function object comp.

   1-3) Constructs an empty container.
   4,5) Constructs the container with the contents of the range [first, last). If
   multiple elements in the range have keys that compare equivalent, it is unspecified
   which element is inserted (pending LWG2844).
   If [first, last) is not a valid range, the behavior is undefined.
   6,7) Copy constructor. Constructs the container with the copy of the contents of
   other.

   If alloc is not provided, allocator is obtained by calling                    (since
   std::allocator_traits<allocator_type>::                                       C++11)
       select_on_container_copy_construction(other.get_allocator()).
   During class template argument deduction, only the first argument contributes (since
   to the deduction of the container's Allocator template parameter.             C++23)

   8,9) Move constructor. Constructs the container with the contents of other using
   move semantics. If alloc is not provided, allocator is obtained by move-construction
   from the allocator belonging to other.

   During class template argument deduction, only the first argument
   contributes to the deduction of the container's Allocator template     (since C++23)
   parameter.

   10,11) Initializer-list constructor. Constructs the container with the contents of
   the initializer list init.If multiple elements in the range have keys that compare
   equivalent, it is unspecified which element is inserted (pending LWG2844).
   12,13) Constructs the container with the contents of rg. If multiple elements in the
   range have keys that compare equivalent, it is unspecified which element is inserted
   (pending LWG2844).

.SH Parameters

   alloc       - allocator to use for all memory allocations of this container
   comp        - comparison function object to use for all comparisons of keys
   first, last - the range to copy the elements from
   other       - another container to be used as source to initialize the elements of
                 the container with
   init        - initializer list to initialize the elements of the container with
   rg          - a container compatible range, that is, an input_range whose elements
                 are convertible to value_type
.SH Type requirements
   -
   InputIt must meet the requirements of LegacyInputIterator.
   -
   Compare must meet the requirements of Compare.
   -
   Allocator must meet the requirements of Allocator.

.SH Complexity

   1-3) Constant.
   4,5) \\(\\scriptsize N \\cdot log(N)\\)N·log(N) where \\(\\scriptsize N\\)N is
   std::distance(first, last) in general, linear in \\(\\scriptsize N\\)N if [first, last)
   is already sorted by value_comp().
   6,7) Linear in size of other.
   8,9) Constant. If alloc is given and alloc != other.get_allocator(), then linear.
   10,11) \\(\\scriptsize N \\cdot log(N)\\)N·log(N) where \\(\\scriptsize N\\)N is
   init.size() in general, linear in \\(\\scriptsize N\\)N if init is already sorted by
   value_comp().
   12,13) \\(\\scriptsize N \\cdot log(N)\\)N·log(N) where \\(\\scriptsize N\\)N is
   ranges::distance(rg) in general, linear in \\(\\scriptsize N\\)N if rg is already
   sorted by value_comp().

.SH Exceptions

   Calls to Allocator::allocate may throw.

.SH Notes

   After container move construction (overload (8,9)), references, pointers, and
   iterators (other than the end iterator) to other remain valid, but refer to elements
   that are now in *this. The current standard makes this guarantee via the blanket
   statement in [container.reqmts]/67, and a more direct guarantee is under
   consideration via LWG issue 2321.

   Although not formally required until C++23, some implementations has already put the
   template parameter Allocator into non-deduced contexts in earlier modes.

       Feature-test macro       Value    Std                   Feature
   __cpp_lib_containers_ranges 202202L (C++23) Ranges-aware construction and insertion;
                                               overloads (12,13)

.SH Example


// Run this code

 #include <iomanip>
 #include <iostream>
 #include <map>
 #include <string>

 template<typename Key, typename Value, typename Cmp>
 std::ostream& operator<<(std::ostream& os, std::map<Key, Value, Cmp> const& map)
 {
     os << "{ ";
     for (auto comma{map.size()}; auto const& p : map)
         os << '\\'' << p.first << "' is " << p.second << (--comma ? ", " : " ");
     return os << "}\\n";
 }

 struct Point
 {
     double x, y;

     friend std::ostream& operator<<(std::ostream& os, Point pt)
     {
         return os << '(' << pt.x << ", " << pt.y << ')';
     }
 };

 struct PointCmp
 {
     bool operator()(const Point& lhs, const Point& rhs) const
     {
         return lhs.x < rhs.x; // NB: y is intentionally ignored
     }
 };

 int main()
 {
     // (1) Default constructor
     std::map<std::string, int> map1;
     map1["something"] = 69;
     map1["anything"] = 199;
     map1["that thing"] = 50;
     std::cout << "map1 = " << map1;

     // (4) Range constructor
     std::map<std::string, int> iter(map1.find("anything"), map1.end());
     std::cout << "\\niter = " << iter;
     std::cout << "map1 = " << map1;

     // (6) Copy constructor
     std::map<std::string, int> copied(map1);
     std::cout << "\\ncopied = " << copied;
     std::cout << "map1 = " << map1;

     // (8) Move constructor
     std::map<std::string, int> moved{std::move(map1)};
     std::cout << "\\nmoved = " << moved;
     std::cout << "map1 = " << map1;

     // (10) Initializer list constructor
     const std::map<std::string, int> init
     {
         {"this", 100},
         {"can", 100},
         {"be", 100},
         {"const", 100}
     };
     std::cout << "\\ninit = " << init;

     std::cout << "\\nCustom Key class option 1:\\n";
     // Use a comparison struct
     std::map<Point, double, PointCmp> mag =
     {
         {{5, -12}, 13},
         {{3, 4}, 5},
         {{-8, -15}, 17}
     };
     std::cout << "mag = " << mag << '\\n';

     std::cout << "Custom Key class option 2:\\n";
     // Use a comparison lambda
     // This lambda sorts points according to their magnitudes, where
     // these magnitudes are taken from the local variable mag.
     auto cmpLambda = [&mag](const Point& lhs, const Point& rhs)
     {
         return mag[lhs] < mag[rhs];
     };

     // You could also use a lambda that is not dependent on local variables, like this:
     // auto cmpLambda = [](const Point& lhs, const Point& rhs){ return lhs.y < rhs.y; };
     std::map<Point, double, decltype(cmpLambda)> magy(cmpLambda);

     // Various ways of inserting elements:
     magy.insert(std::pair<Point, double>({5, -12}, 13));
     magy.insert({{3, 4}, 5});
     magy.insert({Point{-8.0, -15.0}, 17});
     std::cout << "magy = " << magy << '\\n';

     std::cout << "Construction from a range:\\n";
     using PS = std::pair<const std::string, int>;
     const auto rg = {PS{"one", 1}, {"one", 101}, {"two", 2}, {"three", 3}};
 #if __cpp_lib_containers_ranges
     std::map<std::string, int> nums(std::from_range, rg); // overload (12)
 #else
     std::map<std::string, int> nums(rg.begin(), rg.end()); // fallback to (4)
 #endif
     std::cout << "nums = " << nums << '\\n';
 }

.SH Output:

 map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }

 iter = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }
 map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }

 copied = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }
 map1 = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }

 moved = { 'anything' is 199, 'something' is 69, 'that thing' is 50 }
 map1 = { }

 init = { 'be' is 100, 'can' is 100, 'const' is 100, 'this' is 100 }

 Custom Key class option 1:
 mag = { '(-8, -15)' is 17, '(3, 4)' is 5, '(5, -12)' is 13 }

 Custom Key class option 2:
 magy = { '(3, 4)' is 5, '(5, -12)' is 13, '(-8, -15)' is 17 }

 Construction from a range:
 nums = { 'one' is 1, 'three' is 3, 'two' is 2 }

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to             Behavior as published              Correct behavior
   LWG 2076 C++11      overload \fB(4)\fP conditionally required Keyand T   not required
                       to be CopyInsertable into *this
   LWG 2193 C++11      the default constructor was explicit           made non-explicit

.SH See also

   operator= assigns values to the container
             \fI(public member function)\fP
